1. Field of the Invention
The present invention relates to a membrane electrode assembly having an electrolyte membrane which comprises a solid polymer electrolyte membrane and a pair of gas diffusion electrodes which are disposed on both sides of the solid polymer electrolyte membrane, and to a fuel cell unit in which the membrane electrode assembly having an electrolyte membrane is placed between a pair of separators. In particular, the present invention relates to a membrane electrode assembly having a membrane electrolyte membrane and to a fuel cell unit which can improve the sealing ability on an outer side of the solid polymer electrolyte membrane and prevent gas leakage to the outside.
2. Description of Related Art
The fuel cells are known having a structure in which a fuel cell unit is made by sandwiching a membrane electrode assembly, which has an electrolyte membrane comprising a solid polymer electrolyte membrane comprising an anode side gas diffusion layer and a cathode side gas diffusion layer which are disposed on both sides of the membrane between a pair of separators, and in which a plurality of the cell of the fuel cell are stacked.
One example of such a case is explained with reference to FIGS. 10 and 11. In these drawings, reference numeral 1 indicates a membrane electrode assembly having an electrolyte membrane. This membrane electrode assembly 1 having an electrolyte membrane comprises a solid polymer electrolyte membrane 2 and gas diffusion layers (an anode side gas diffusion layer and a cathode side gas diffusion layer) 3 and 4 which are disposed on both sides of the membrane. A catalyst layer is disposed between the solid polymer electrolyte membrane 2 and each of the gas diffusion layers 3 and 4. The solid polymer electrolyte membrane 2 is formed so as to have planar dimensions that are larger than planar dimensions of the anode side gas diffusion layer 3 and the cathode side gas diffusion layer 4 which are disposed on both sides thereof, and the solid polymer electrolyte membrane 2 extends over a periphery of the diffusion layers 3 and 4. A pair of separators 5 and 6 are disposed on both sides of the membrane electrode assembly 1 having an electrolyte membrane as shown in FIG. 11. Round-shaped seal members 7 are set at the peripheral edge side of the opposing surfaces of separators 5 and 6; thus, the solid polymer electrolyte membrane 2 is sandwiched between the round-shaped seal members 7. A fuel cell 8 is structured by disposing the membrane electrode assembly 1 having an electrolyte membrane between the separators 5 and 6 in such a state. In addition, in the separators 5 and 6, gas communication channels 9 and 10 for supplying fuel gas, oxidant gas, and cooling medium communication channels 11 for supplying cooling medium are formed.
In the fuel cell 8 which is structured in the above manner, when a fuel gas (for example, hydrogen gas) is supplied to a reaction surface of the anode side gas diffusion layer 3 via the gas communication channels 9, the hydrogen is ionized by the catalyst layer and transmitted to the cathode side gas diffusion layer 4 via the solid polymer electrolyte membrane 2. Electrons which are generated during such a process are taken to an external circuit and used for direct current electric energy. Because an oxidant gas (for example, a gas including oxygen) is supplied to the cathode side gas diffusion layer 4, hydrogen ions, electrons and oxygen react there, and a water is generated.
However, in the solid polymer electrolyte membrane 2, as shown in FIG. 10, a part of the solid polymer electrolyte membrane 2 extends from the peripheries of the diffusion layers 3 and 4. Therefore, if there is any difference in the positions of the opposing seal members 7 which have the solid polymer electrolyte membrane 2 therebetween, a shearing stress acts thereon; therefore, there is a possibility that the solid polymer electrolyte membrane 2 may be damaged. Also, when a fuel gas and an oxidant gas are supplied to a cell of the fuel cell 8, a difference of pressure 12 (pressure difference between the electrodes) occurs between the one side and the other side of the solid polymer electrolyte membrane 2; thus, as shown in FIG. 12, there is a possibility that warping 13 occurs in the solid polymer electrolyte membrane 2. Therefore, it is necessary very precisely to perform the positioning operation of the opposing seal members 7 which have the solid polymer electrolyte membrane 2 therebetween. There is a problem in that it takes a lot of time to produce a cell of the fuel cell 8 while maintaining the required precision; thus, it is burdensome from the viewpoint of efficient production.
Regarding this aspect, a fuel cell unit 20 having a structure shown in FIG. 13 is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 10-289722. That is, a gas diffusion layer 25 having nearly the same dimensions as a solid polymer electrolyte membrane 22 is disposed on an opposite surface of a catalyst layer 23 which contacts the solid polymer electrolyte membrane 22 so as to form a unit with the catalyst layer 23, and the peripheral edge section of the solid polymer electrolyte membrane 22 is reinforced by contacting the gas diffusion layer 25 to the peripheral edge section of the solid polymer electrolyte membrane 22. Also, a seal member 26 having a C-shaped cross section on one side is disposed so as to cover an end section of the gas diffusion layer 25; thus, prevention of gas leakage to the outside is intended.
However, in the cell of the fuel cell 20, because it is necessary to form the gas diffusion layer 25 so as to be nearly the same size as the solid polymer electrolyte membrane 22, there is a problem in that the material procurement cost increases due to the extra cost for material procurement. Also, the size of the gas diffusion layer 25 is different from the size of another gas diffusion layer 25b; thus, the number of parts increases, and the production process becomes complicated.
Furthermore, in the cell of the fuel cell 20, there is a problem in that it is burdensome to form the seal member 26 so as to fit to the shape of the gas diffusion layer 25 and to assemble the seal member 26 so as to cover the gas diffusion layer 25 because the seal member 26 having a C-shaped cross section is disposed so as to cover an end section of the gas diffusion layer 25.